As an example of a conventional electromagnetic transducer, there has been provided an electromagnetic transducer in which a permanent magnet plate is arranged to be opposed to a vibrating membrane, a shock absorbing material is placed between the permanent magnet plate and the vibrating membrane as needed, and the whole of the electromagnetic transducer is covered by a frame and formed into a rectangular shape. The permanent magnet plate used in this example has beltlike magnetization portions which are arranged at a fixed pitch and which have opposite magnetic pole orientations alternately. Furthermore, on a film surface of the vibrating membrane, a serpentine tangible pattern acting as a magnetic coil is formed in a portion which is called a magnetization neutral zone in such a way that two lines of the tangible pattern extend to be opposed to each other (for example, refer to patent reference 1). When a current of an audio signal is made to flow through the coil pattern formed on the vibrating membrane, the conductive pattern acting as a magnetic coil is electromagnetically coupled with the magnetization pattern of the permanent magnet plate, and the vibrating membrane having the above-mentioned conductive pattern vibrates according to the Fleming's law. A sound wave caused by this vibration is emitted out via a sound hole bored in the permanent magnet plate and a sound hole bored in the frame. In other words, the electromagnetic transducer carries out audio reproduction as a speaker.
Furthermore, there has been provided an ultra-thin speaker having the same structure as the above-mentioned electromagnetic transducer, i.e. a “Gamuzon type speaker” (for example, refer to nonpatent reference 1). This type of speaker is provided with a permanent magnet plate formed of rod-shaped block magnets, and its other components are the same as those of the conventional electromagnetic transducer shown above. The rod-like magnets are constructed and arranged in such a way that plural pairs of rod-like magnets having the same magnetic pole orientation (i.e. the north (south) poles of the two rod-like magnets in each pair are oriented in the same direction) are aligned in a direction perpendicular to the rod-like magnets with the magnetic pole orientations of the plural pairs of rod-like magnets being varied alternately. The electromagnetic transducer having this structure carries out generation of sound during audio reproduction in the same way that the example shown in the beginning of this section does.    [Patent reference 1] Japanese patent No. 3192372 gazette    [Nonpatent reference 1] Speaker & enclosure encyclopedia, Sections 2 to 25, compiled under the supervision of Tamon Saeki, Seibundo Shinkosha (issued in May 1999)
A problem with either of the conventional electromagnetic transducers as mentioned above is that it is difficult to provide a vibrating membrane that vibrates with a large amplitude, and therefore the sound pressure level of sound being played back in a low-pitched sound region is low. The main cause is the difficulty of enlarging the gap between the opposing permanent magnets in each pair. The reason why it is difficult to enlarge the gap between the opposing permanent magnets in each pair is because simple increase in the gap causes reduction in the magnetic flux density at a position of the coil pattern (i.e. a position of the vibrating membrane) which produces a driving force. Furthermore, because simple increase in the thickness of each magnet in order to increase the magnetic flux density causes increase in the magnetic flux density in the vicinity of the surface of each magnet, and the larger amplitude the vibrating membrane has, i.e. the nearer to the surface of each magnet the vibrating membrane is positioned, the larger driving force is generated, the vibrating membrane comes in contact with the permanent magnets and this results in a cause of creating a sound distortion and abnormal noise.
The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to provide an electromagnetic transducer that enables a low-pitched sound reproduction at a high-volume level.